Calcium aluminate hydraulic cements were first prepared in Europe during the latter part of the nineteenth century. Although developed initially to take advantage of their chemical resistance to sulfate, saline, and low pH solutions, the primary property which distinguishes calcium aluminate cements from common portland cement (calcium silicate-based) is their much more rapid rate of strength development. For example, typical times for maximum strength development are on the order of several days for portland cement in comparison with several hours for high alumina compositions.
In the 1920's it was discovered that mortars and concretes prepared with a calcium aluminate cement and a refractory aggregate such as alumina or fireclay, did not crack or spall during repeated firings, and could withstand temperatures in the range of about 1400.degree.-1700.degree. C. Those characteristics, in conjunction with good field workability (castability), high hot and cold strength, and rapid hardening, have led to a castable refractories industry which today constitutes the principal application for high alumina, calcium aluminate cement concretes. The steel, glass, non-ferrous, and ceramics industries are major users.
From the post World War II period to the late 1960's, high alumina, calcium aluminate cement concretes were utilized increasingly for load bearing, pre-cast beams and slabs. Inasmuch as the time to achieve maximum strength was substantially reduced with those cements, overall fabrication costs were lower than with portland cement. This rapidly expanding industry came to an abrupt end, however, when, in a period of ten years from the early 1960's to the early, 1970's, several catastrophic building collapses occurred on the European continent. Extensive investigations into those disasters traced the structural failures to a gradual, but drastic, weakening of the cement concrete through a process now termed "conversion". As a direct result of those investigations, high alumina, calcium aluminate cement has been banned for structural applications in virtually every country in the world.
The sequence of chemical reactions underlying the conversion of metastable hydrates (principally CaO.multidot.Al.sub.2 O.sub.3 .multidot.10H.sub.2 O and 2 CaO.multidot.Al.sub.2 O.sub.3 .multidot.8H.sub.2 O) to the stable hydrate (3CaO.multidot.Al.sub.2 O.sub.3 .multidot.6H.sub.2 O+Al.sub.2 O.sub.3 .multidot.3H.sub.2 O) generally proceeds as follows:
(1) CaO.multidot.Al.sub.2 O.sub.3 (cement)+10H.sub.2 O.fwdarw.CaO.multidot.Al.sub.2 O.sub.3 .multidot.10H.sub.2 O PA0 (2) 2(CaO.multidot.Al.sub.2 O.sub.3 .multidot.10H.sub.2 O).fwdarw.2CaO.multidot.Al.sub.2 O.sub.3 .multidot.8H.sub.2 O+Al.sub.2 O.sub.3 .multidot.3H.sub.2 O+9H.sub.2 O PA0 (3) 3(2CaO.multidot.Al.sub.2 O.sub.3 .multidot.8H.sub.2 O).fwdarw.2(3CaO.multidot.Al.sub.2 O.sub.3 .multidot.6H.sub.2 O)[Hydrogarnet]+2(Al.sub.2 O.sub.3 .multidot.3H.sub.2 O)[Gibbsite]+6H.sub.2 O
In conformity with the standard abbreviations of the cement chemist, the following symbols will be employed hereinafter:
C=CaO; A=Al.sub.2 O.sub.3 ; S=SiO.sub.2 ; H=H.sub.2 O
When it is appreciated that there is a decrease in specific volume of over 50% (excluding H.sub.2 O) during the transformation of CAH.sub.10 to C.sub.3 AH.sub.6, it is quite apparent that the observed weakening phenomenon is effected primarily through an increase in porosity occurring during the conversion process.
The hydrogarnet structure is the sole calcium aluminate hydrate recognized to be stable under ambient conditions. That phase occurs in nature as "hydrogrossular" (C.sub.3 AH.sub.6) and is isostructural with cubic grossular(ite) (C.sub.3 AS.sub.3) garnet. A hydrothermal-x-ray study of the hydrogrossular-grossular garnet series has indicated that a continuous solid solution series exists between C.sub.3 AH.sub.6 and C.sub.3 AS.sub.3 with the general formula C.sub.3 AS.sub.x H.sub.(6-2x). A substitution of 4H+ ions for the Si.sup.+4 ion in the SiO.sub.4 tetrahedron comprises the mechanism to drive that solid solution.